net: Remove unnecessary driver assignments of ethtool_ringparam fields to zero

Per comments from Ben Hutchings on a previous patch, sweep the floors
a little removing unnecessary assignments of zero to fields of struct
ethtool_ringparam in driver code supporting ethtool -g.

Signed-off-by: Rick Jones <rick.jones2@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>